DESCRIPTION (Verbatim from applicant's abstract): The long-term objective of this application is to understand the control of normal variation in the cytoarchitecture of the major retinal cell populations in adult mice. The results of this application will describe variation in rod and cone photoreceptor number and density as well as variation in the dorsoventral fields of S-, M-, and double-pigmented cones in different strains, and will identify the quantitative trait loci (QTL) controlling these phenotypes. Photoreceptor number will be acquired with stereologically unbiased counting methods from both retina wholemounts using DIC optics and from stained cryostat-sectioned retinas. Cone subtypes will be distinguished by immunocytochemical staining. Understanding retinal development, particularly photoreceptor development, has clear health benefits, because many human diseases resulting in blindness affect photoreceptors.